Report, TSKS03 Wireless Systems - Linköping University · 2016-05-16 · of inherent data security...

1 TSKS03 Wireless Systems VT2016

Report, TSKS03 Wireless SystemsInfrared Communication

Kashyap Malthish, kasma064

May 9th, 2016

Kashyap Malthish [email protected]


Contents

Acronyms, Tables and Figures 3-4

1 Abstract 5

2 Introduction 5

2.1 IR Configurations 5

2.2 IR Communication Channel 6

3 IR Transceiver Design 7

3.1 Noise Sources, BER and Sensitivity 9

3.2 Optical Sources 9

4 Modulation Schemes 10

4.1 Multiple Access Techniques 13

5 IrDA Protocols 14

6 Limitations 17

7 Conclusion 18

8 References 19



AcronymsFOV Field of View

LOS Line of Sight

Non-LOS Non-Line of Sight

IrDA Infrared Data Association

AWGN Additive White Gaussian Noise

D-PIM Digital Pulse Interval Modulation

PPM Pulse Position Modulation

IM/DD Intensity Modulation/Direct Detection

PIN Positive-Intrinsic-Negative Diode

CMU Clock Multiplier Unit

CDR Clock and Data Recovery

RMA Receiver Main Amplifier

OOK On-Off Keying

DPPM Differential Pulse Position Modulation

MLSD Maximum Likelihood Sequence Detection

WDMA Wavelength Division Multiple Access

SDMA Space Division Multiple Access

BER Bit Error Rate

SIR Standard Infrared

MIR Medium Infrared

FIR Fast Infrared

VFIR Very fast Infrared

UFIR Ultra fast Infrared

GigaIR Infrared in Gigabits

ISI Intersymbol Interference



EGC Equal Gain Combining

MLC Maximum Likelihood Combining

SD Selection Diversity

MRC Maximum Ratio Combining

TDMA Time Division Multiple Access

CDMA Code Division Multiple Access

List of Figures

Figure 1: Various Possible IR Configurations 6

Figure 2: A Simple Channel Model 7

Figure 3: Block of an optical Wireless Link 8

Figure 4: Transceiver Schematic 9

Figure 5: Sources of Noise in a Receiver 10

Figure 6: Pulse Position Modulation Scheme 10

Figure 7: 4-PPM and 4-DPMM Scheme 11

Figure 8: L-PPM Schemes 12

Figure 9: OOK with NRZ and RZ Pulse

Figure 10: SDMA Technique 13

Figure 11: IrDA Protocol Stack 14

List of Tables

Table 4.1: Mapping of source into 4-PPM and 4-PIM 12



1 Abstract

This report describes about infrared communication technology in the context of wirelesscommunication systems. The sections below provide an overview of infrared communicationincluding a brief introduction followed by an explanation of transceiver design, modulationschemes, IrDA protocols and a brief discussion on some of the limitations of infraredcommunication technology.

2 Introduction

Infrared communication is a form of optical communication that has been around for a long timenow. One of the earliest use of infrared technology in wireless communication was the remotecontrol for television and Video Cassette Recorders (VCRs). The Infrared Data Association (IrDA)cites that the application of infrared communication for television remote control was explored inthe 1980s. But the first experiment of examining the feasibility of IR communication wasperformed in the 1970s when some computer terminals were connected with an IR transceiver overa short distance. Most of the remote controls for DVD players, Satellite to home boxes for TVstoday are based on infrared communication technology. Since IR communication involvestransmitting data in a wireless manner, the technology as a whole, is now bracketed under wirelesscommunication systems. As will be seen in further sections, the application of IR communicationtechnology is not restricted to just a remote control, but extends to various indoor and outdoorapplications; specifically, in many entertainment devices and in the field of health care monitoringas well.

Within the domain of wireless communication systems, the key advantages offered by IR are: easeof use, small size, low cost implementation and capability to offer huge bandwidths which isunregulated worldwide. IR is also immune to electromagnetic interference and offers a certain levelof inherent data security as IR does not go through the walls.

2.1 IR Configurations

Two important parameters define the sort of wireless IR link that can be configured in an IRcommunication system as illustrated in Figure 1. They are:

• Line of Sight (LOS) or Non Line of Sight Configuration.

• Directed, non-directed or hybrid configuration in the transmitter and receiver.

The transmitter/receiver are unobstructed and view each other directly in the LOS configuration. ANon-LOS configuration takes into account the possibility of an obstructive path between thetransmitter and the receiver, thus reflective surfaces are widely used to reduce multipath distortion.



Figure 1: Various Possible IR Configurations [1]

If the transmitter and the Field of View (FOV) of the receiver are restricted, then the configurationis direct. When both the transmitter and receiver have a broad emission range and wide FOVrespectively, they are classified as non-directed systems, as illustrated in Figure 1-c. When either thetransmitter or receiver is restrictive, then the configuration is termed hybrid, shown in Figure 1-f.

A popular configuration is the diffuse configuration, which means that it is non-directed and non-LOS. In the diffuse configuration the transmitter and the receiver employ wider beam emission andthe field of view respectively. If the wall surface and ceiling within the room are reflective, then auniform spread of the IR energy, with lesser reflection loss can be ensured. This configurationresults in a more portable and robust IR system. Figure 1 shows the two popular diffuse and quasidiffuse IR configurations.

2.2 IR Communication Channel

Intensity modulation with direct detection is the method used for data transmission and reception in



a vast majority of applications which use IR communication. With the help of a Light EmittingDiodes (LEDs) or a Laser diodes (LDs) the electrical signal data can be modulated onto an opticalIR carrier. The detector usually employs a photodetector which operates by converting the receivedoptical power to an equivalent current. Background illumination is the deciding factor for modelingthe optical channel. Signal at the receiver is modeled as Poisson process for low backgroundillumination and as Additive White Gaussian Noise (AWGN) for higher background illumination.Some of the common sources of background illumination are fluorescent lamps in indoor systemsand sun light in the case of outdoor systems.

Figure 2: A simple channel model [1]

The channel model for a IM/DD wireless system can also be described by the equation:

where x(τ) is the transmitter optical signal, n(t) is the Gaussian noise, h(t) is the multipath impulseresponse. If the channel is to be defined in terms of its frequency response then the correspondingtransfer function is given by:

3 IR Transceiver Design

The design of a transmitter involves several constraints. The key constraint being eye safety. This



results in restriction in the amount of power that can be transmitted. The key issue of backgroundillumination has to be considered while designing the receiver section. The block diagram of thetransmitter/receiver section is shown below in Figure 3.

Figure 3: Block of an Optical Wireless Link [1]

The electrical signal is converted to an optical signal at the transmitter side. This task isaccomplished by the drive circuit in the transmitter with the help of a suitable light source such as alaser diode or a light emitting diode. The receiver end usually consists of a photodetector whichuses either a positive-intrinsic-negative(PIN) diode or the avalanche photodiode (APD). Both thetransmitter and receiver are implemented on an integrated circuit, as a transceiver. The blockdiagram of such a system is shown below in Figure 4. A MUX combines the digitized data into asingle data stream. A clock multiplication unit (CMU) is then used to combine a slower word clockto a faster bit-rate-clock for improving the data timing. This is followed by an optical drive portionwhich consists of the laser driver and a modulator driver which work towards data modulation.Design of the receiver is dependent on the modulation scheme implemented on the transmitter side.The receiver block performs the reverse action. When the photodiode receives the opticalinformation, it converts the information into a current. The preamplifier section amplifies andconverts the current to a suitable voltage. Further voltage amplification is performed by receivermain amplifiers

Figure 4: Transceiver Schematic [1]



(RMA). The received data information is timed again by the clock and data recovery circuit (CDR) and fed into a demux, which converts serial data into parallel data for further processing.

3.1 Noise sources, sensitivity and BER

Noise is an inevitable component and plays a role in IR communication as well. The sensitivity ofan IR receiver is dependent on the noise. The concepts of noise figure/factor used in RFcommunication can be conveniently applied in IR communication. Noise at the receiver end couldeither be signal dependent or independent. Figure-5 below shows the various sources of noise. Thekey sources of noise are the following three:

• Noise contribution from the optical source.

• Noise contribution due to atmospheric turbulence and background illumination.

• Noise contribution from the receiver block.

Within IR communication links, sensitivity and bit-error rate are considered to be inter dependent.Sensitivity is an indicator of reliability of the receiver to detect optical data in spite of atmosphericattenuation on the optical data. Usually, bit error rate is performance measure in digitalcommunication. BER plot is usually employed to study the performance. The BER is plotted eitheras a function of the received power or the amplifier input current. The bandwidth of a receiver isfixed at around 65% of the bit rate.

Figure 5: Sources of noise in a receiver [1]

3.2 Optical Sources

Optical source forms an essential part of an optical communication system. High intensity and goodmonochromaticity are the two factors which decide the type of light source for communication.Laser diodes and LEDs are the most commonly used light sources. The choice of LDs or LEDs areapplication dependent. When speed is deemed necessary, LDs are preferred over LEDs. Inapplications where temperatures could play a crucial role, LEDs are preferred over LDs.



4 Modulation Schemes

The overall system performance is paramount in the choice of modulation technique. The variousmodulation techniques are characterized by a certain bandwidth and efficiency(high speedtransmission). Pulse Position Modulation (PPM) and on-off keying are two most popularmodulation techniques employed in infrared communication systems. The modulation techniquegenerally involves converting the analog signal into samples and these samples are transmitted withthe help of pulses that are modulated in terms of the position with reference to the analog signalamplitude. Three variants of pulse modulation schemes have been tried.

Figure 6: Pulse Position Modulation Scheme

L-PPM, where there are 'L' slots in a single symbol, called chips. Only one chip slot carries theoptical power, whereas the other L-1 chips will not transmit power. It has been seen that L-PPMtechnique has some drawbacks due to Intersymbol Interference (ISI) and as such MaximumLikelihood Sequence detection, equalization, rate 2/3 trellis coded 8-PPM, rate 3/4 trellis coded 16-PPM techniques are used to overcome ISI problem. The different schemes are shown in Figure 8.

D-PPM, also called Differential PPM is a technique used when multipath distortion is expected to

Figure 7: a) 4-PPM Scheme b) 4-DPPM Scheme [5]

be low. This is typically used in direct LOS configurations. Consider a 4-PPM symbol with fourchips. Usually, one of the chip is high and the remaining three chips are low. A D-PPM can beobtained by removing the low chips that follow a high chip. MLSD technique is used for symboldecoding as the symbol boundaries are not known previously. The schemes are shown in Figure 7.



Figure 8: L-PPM Schemes [1]

D-PIM, also known as digital pulse interval modulation is another technique where the input signalamplitude determines the intervals between the narrow pulses and each successive time framecommences immediately after the previous pulse. Based on the content of the symbol, the length ofthe symbol varies. It is possible to achieve higher bit rates using D-PIM than using L-PPM. Thetable below shows the mapping of source bits and chips for transmission with 4-PPM and 4-PIM.



Source Bits 4-PPM Chips 4-PIM Chips

00 1000 1(0)

01 0100 1(0)0

10 0010 1(0)00

11 0001 1(0)000Table 4.1: Mapping of source into 4-PPM and 4-PIM [3]

On-off Keying(OOK) is another popular technique which is used for modulation in IRcommunication due to the ease of implementation. OOK also offers good bandwidth efficiency. Inprinciple, OOK is a form of Amplitude Shift Keying (ASK) where the carrier amplitude is increasedfor '1' transmission and the carrier amplitude is decreased for '0' transmission. Since OOK is avariant of ASK, the additional feature is that the carrier amplitude being completely OFF for a '0'transmission. Some authors have discussed using both return-to-zero(RZ) pulses and non-return-to-zero(NRZ) pulses for representation; but with a higher preference for the latter technique.

Duty cycle for the pulses could change. When the value of 'gamma' is < 1, then average power isdecreased and bandwidth factor is increased by 1/gamma. OOK receiver could either be acontinuous time matched filter, or a whitened matched filter based on whether multipath distortionis taken into consideration or not. The waveforms for the NRZ and RZ schemes are shown in thetwo equations and in Figure-9 shown below.

where pTb(t) = 1 for 0 ≤ t ≤ Tb and pTb(t) = 0 elsewhere, and a belongs to {0, 1}. Here, the peak

optical power Ppeak is related to the average optical power Pavg as follows: Ppeak = 2Pavg.

Figure 9: a) OOK with NRZ pulse b) RZ pulse [6]



4.1 Multiple Access Techniques

The need for multiple access techniques arise due to constraints like limited hardware in atransceiver system and limited spectrum availability. In the context of IR communication, multipleaccess techniques describe the ways of allocating access to the channel for different users. Thesetechniques are mostly defined at the physical layer of IR communication system. Two popularmethods for multiple access are: 1) Electrical Multiple Access and 2) Optical Multiple Access.

Optical features of IR are managed to allocate access to the users. Two methods of optical multipleaccess techniques that have been used in IR are Wavelength Division Multiple Access (WDMA)and Space Division Multiple Access (SDMA).

Figure 10: SDMA Technique implementation [4]

In the WDMA scheme, users transmit information at different wavelengths. This is similar toFrequency Division Multiple Access (FDMA), except that WDMA relates to optical signals.Realizing a WDMA transceiver is relatively simple, with the use of just a laser diode with narrowline-width and multiple photodetectors at the receiver end to detect wavelengths. In complexapplications, using a WDMA system results in higher cost as the number of terminals operating atvarious wavelengths increases. The equation for interference model is shown below.

Xj(t), j= 1,2,…, N is the transmissions that are incident on the receiver. hj is the channel impulseresponse between the jth transmiter and the receiver. Y(t) denotes the photocurrent. Studies alsoshow that performance of WDMA scheme degrades because of Optical Beat Interference (OBI).This phenomenon occurs when the two or more of the nearly similar wavelengths are incident on



the receiver. Some solutions to overcome this problem are:

• Over-modulation of the laser transmitter.

• Using Forward Error Correction (FEC) with out-of-band clipping tones.

• Modulating every laser transmission with large CDMA signal.

When using WDMA system with multiple terminals, the overall cost and complexity of the systemincreases. This is because of the need for tunable lasers at the transmitter end and high precisiontunable band pass filters at the receiver end to detect the various wavelengths. Space DivisionMultiple Access (SDMA) offers an alternative. SDMA technique involves using an Angle DiversityReceiver (ADR) and detects signals from multiple directions. In combination with a variety ofquasi-diffuse transmitters, ADRs can be configured for a specific application. ADRs use a numberof techniques like Equal Gain Combining (EGC), Maximum Likelihood Combining (MLC)Selection Diversity (SD) and Maximum Ratio Combining (MRC) for detection and processing thereceived signals. SDMA techniques using LOS transmitters and quasi-diffuse transmitters areshown in Figure 10.

Among the electrical multiple access techniques, Time Division Multiple Access (TDMA) andCode Division Multiple Access (CDMA) are mostly used in IR communication. Transmission inTDMA occurs in various time slots. Better power efficiency is achieved due to low duty cycle. Thedisadvantage with TDMA is system complexity as a consequence of high level of coordinationrequired for good synchronization. In CDMA there is simultaneous transmission since orthogonal orquasi-orthogonal codes are used by multiple users. Various multiple access schemes in combinationwith modulation techniques have been studied. These include TDMA with 2-PPM, TDMA with 4-PPM, TDMA with OOK, CDMA with Optical Orthogonal Codes (OOC), CDMA with m-sequences, and FDMA with BPSK.

5 IrDA® Protocols

There is a variety of protocols available for wireless communication. Protocols exist for both RF aswell as infrared communication, the popular ones for RF communication being the IEEE 802.11protocol and Bluetooth. Protocols for infrared communication have been developed and specifiedby the Infrared Data Association®, also called IrDA®. IrDA® works towards a commoninteroperability standard between the various components, peripherals, hardware and software forusing infrared communication. The association is composed of: the architectural council andSpecial Interest Groups (SIGs) which are involved in protocol development and specifications; andvarious committees which look into exploring new market opportunities and provide servicing.



Figure 11: IrDA Protocol Layers [2]

The protocol stack is shown in the Figure 11. The stack arrangement is in a layered manner with thebottom most tier being the physical layer. The various stack layers are explained below.

• Physical Layer: This is the bottom most layer in the protocol and provides definitions for thephysical components used in IR communication. Manufacturers are to follow compliancewith certain optical, physical and electrical characteristics for both the transmitter andreceiver design. Generally, the characteristics would be operation wavelengths, data rates,FOVs for emission and detection, rise and fall times, bit error rates (BERs), pulse durationand range. The April 1994 spec release supported 115.2 kbps and the current release aims at5-10 Gbps. The different transceivers in the physical layer are listed below:

➢ SIR: 9.6–115.2 kbit/s, asynchronous, RZI, UART-like, 3/16 pulse

➢ MIR: 0.576–1.152 Mbit/s, RZI, 1/4 pulse, HDLC bit stuffing

➢ FIR: 4 Mbit/s, 4PPM

➢ VFIR: 16 Mbit/s, NRZ, HHH(1,13)

➢ UFIR: 96 Mbit/s, NRZI, 8b/10b

➢ GigaIR: 512 Mbit/s – 1 Gbit/s, NRZI, 2-ASK, 4-ASK, 8b/10b

➢ 5/10GigaIR: seems to be a new IrPHY coming soon



Some further characteristics are mentioned below.

Range: standard 1 m; low-power to low-power 0.2 m; standard to low-power 0.3 m. The 10GigaIR also define new usage models that supports higher link distances up to severalmeters.

Angle: minimum cone ±15°

Speed: 2.4 kbit/s to 1 Gbit/s

Modulation: baseband, no carrier

Wavelength: 850–900 nm

• Frame and Driver: This is the software layer in the protocol stack. The components whichare used in the physical layer would interact with the upper layers with the help of softwareand hardware which is defined within this layer. The framer converts data into frames fortransmission purpose. It consists of boundary bytes, synchronization bytes and errordetection bytes. The controller is defined at both the transmitter and receiver. At thetransmitter, it drives the optical source based on the data from the stack. At the receiver side,it converts the data into an understandable form for the layers in the stack. The driver isresponsible for controller initialization and information transfer.

• IrLAP: This layer is similar to the data link layer in OSI architecture. Some functions of thislayer are access control, discovery of potential communication partners, establishing abidirectional connection, distribution of the primary/secondary device roles, negotiation ofQoS parameters

• Link Management Protocol: With the help of IrLAP layer, this layer is responsible for accessto various application and services.

• Information Access Protocol: This layer is responsible for discovery, registration and serviceaccess.

• Tiny TP Protocol: This layer manages flow control at link management protocol layer forsegmentation and reassembly (SAR) for efficient channel use.

• Session and Application Protocol: This layer speceifies rles of exchange. The protocolsinclude IrOBEX, IrComm, IrWW, IrTran-P, and IrLAN.



6 Limitations

Some of the limitations faced by IR communication technology are listed below. Factor such aswhether the system is installed for indoor or outdoor application goes on to introduce certainlimitations in the data rate. They are as follows:

• Atmospheric Disturbances: The effects of atmospheric disturbances are more pronouncedwhen the system is used in an outdoor environment. Certain conditions such as fog and hazecan greatly hinder the performance. Attenuation from fog and haze could reach upto300dB/km. A similar effect is seen when there is smoke in the surroundings. Evencommunication between a distance of 100m becomes extremely difficult in the abovesituations. Studies have shown that rain and snow do not have much effect on the datacommunication. In addition to all the above effects, scintillation and air absorption areknown to cause problems for long distance transmissions. Use of novel adaptive opticaltechniques, coding and equalization have been studied and when applied, have shown tomitigate the atmospheric disturbances.

• Eye Safety: While it is true that higher optical power at the transmitter end would improvethe system SNR, eye safety considerations are paramount, hence there is a limitation in themaximum transmitted power. Medical studies confirm that near-IR radiation above athreshold could cause retinal burns and medium/far-IR radiation could cause corneal burns.This is one of the major constraints in IR communication.

• Noise due to Ambient Illumination: This is one of the unavoidable contributor to introducingdata rate limitations since the IR transmitter could be exposed to light from lamps in theindoors or from the sun in the outdoors which introduce noise in the detector at the receiverend of the system and result in a diminished SNR. Two techniques are commonly employedto overcome the issue of noise due to ambient light source. Firstly, a narrow FOV receiverfollowed by the use of optical filters (either bandpass or longpass).

• Multipath distortion: This effect is mainly seen in diffuse configuration. Since thisconfiguration employs an expanded receiver FOV, there are now multiple paths for theenergy. Further a phenomenon called temporal dispersion also leads to ISI.



7 Conclusion

This work has explained the concept of infrared communication within the domain of wirelesscommunication systems. Infrared Communication along with radiofrequency communication canplay an important role in wireless systems. With features like higher bandwidths, low cost, bettersecurity features, smaller size and low power consumption, infrared communication offers someadvantages compared to radio communication. This is even more pronounced in a hospital settingwhere interference of RF communication with medical equipments such as a pacemaker could leadto fatal consequences. So in a hospital setting IR communication is clearly advantageous.

But IR communication is not without drawbacks. As seen in the previous section, issues such asattenuation due to blocking/obstruction, disruption in the link, background illumination noise andother atmospheric limitations, eye safety affect the quality of data communication using infraredtechnology. This leads to the question of whether IR communication could take the place of RFcommunication. This is both debatable and doubtful. IR communication can only complement RFcommunication in certain settings. Currently, studies are underway to develop an integrated hybridsystem incorporating both RF as well as IR communication. Such a hybrid would exploit thebandwidth benefits of IR. Such a hybrid system would also include a handover mechanism betweenRF and IR for throughput optimization. A benefit of this design is compatibility with existing RFdevices and improved scalability. Such systems are presently being developed for indoorenvironments prone to data congestion. NASA has developed a two way Lunar LaserCommunication Demostration (LLCD) using near infrared for interplanetary probes. LLCD is aspace terminal that transmits and receives data between the moon and the ground station. Two waycommunication has also been developed in Satellite to Home boxes and its remotes. Theimplementation is in a way such that data is received by the remote from the boxes, which can thenused for updating the software. The same channel could be used for communication between TV,box and the remote.

Further, IrDA is also working towards improving and compensating certain drawbacks in IRtechnology. The biggest factor that works in favor of IR communication is economic feasibility, sowith a number of research experiments being carried out in the field of IR technology, we wouldcertainly see IR communication introduced in additional settings in the near future.



References

[1] Roberto Ramirez-Iniguez, Sevia M. Idrus, and Ziran Sun, ”Optical wireless communication forIR for wireless connectivity”, Boca Raton,CRC Press, 2008

[2] https://en.wikipedia.org/wiki/Infrared_Data_Association

[3] Marijan Herceg, Tomislav Svedek, and Tomislav Matic, "Pulse Interval Modulation for Ultra-High Speed IR-UWB Communications Systems", EURASIP Journal on Advances in SignalProcessing, 2010, Volume 2010, Number 1, Page 1.

[4] Joseph M. Kahn, Roy You, Pouyan Djahani, Amy G. Weisbin, Beh Kian Teik, and Andrew Tang,"Imaging Diversity Receivers for High-Speed Infrared Wireless Communication", IEEECommunications Magazine, pp. 88–94, December 1998.

[5] Da-shan Shiu, and Joseph M. Kahn, "Differential Pulse-Position Modulation for Power-EfficientOptical Communication", IEEE Transactions on Communication, Vol. 47, No. 8, pp. 1201-1210,August 1999.

[6] J.M. Kahn and J.R. Barry, Wireless Infrared Communications, Proceedings of the IEEE, 85(2),265–298, 1997

[7] http://www.space.com/22680-nasa-lunar-laser-communications-experiment-infographic.html

[8] Rahaim, M. B., Vegni, A. M., and Little, T. D. C., “A hybrid radio frequency and broadcastvisible light communication system,” in IEEE Global Communications Conference (GLOBECOM2011) Workshops, Dec. 5-9, 2011, pp.792-796


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Report, TSKS03 Wireless Systems - Linköping University · 2016-05-16 · of inherent data security...

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